Abjad is the technical term for the type of writing system used by Semitic languages (Hebrew, Arabic, etc.), where there are glyphs for all the consonants but the reader must be prepared to guess what vowel to add between two consonants.Both Hebrew and Arabic have optional vowel marks and are called impure abjads. Ancient Phoenician had nothing but consonants and is a pure abjad. See Also: alphabet, abugida, syllabary and the relevant Wikipedia article.
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